Anticorrosive surface coatings

ABSTRACT

Compounds of formula I: ##STR1## in which R, R 1 , R 2 , R 3 , R 4 , R 5 , R 6  and R 7  are as defined in claim 1, are suitable as corrosion inhibitors in surface coatings for metals.

This application is a continuation of application Ser. No. 510,385,filed Apr. 17, 1990 abandoned.

The invention relates to surface coatings containing S-benzylatedderivatives of 2-mercaptobenzothiazole as corrosion inhibitors.

The use of 2-mercaptobenzothiazole and its salts as corrosion inhibitorsfor surface coatings is known e.g. from EP-A-3817. Various S-substitutedderivatives of mercaptobenzothiazole have also been proposed for thispurpose, e.g. carboxylic acid derivatives (EP-A-128 862) and phenolicderivatives (EP-A-259 255). S-Benzyl derivatives ofmercaptobenzothiazole have been proposed as anticorrosive additives foracid metal pickling baths (JP-A-85/141 879). Metal pickling baths areaqueous solutions of acids and an added corrosion inhibitor must act atthe water/metal interface. By contrast, a corrosion inhibitor in surfacecoatings must act at the binder/metal interface, which demands quitedifferent physicochemical properties. It was therefore surprising tofind that such S-benzyl derivatives of mercaptobenzothiazole also havean outstanding anticorrosive action on the metallic substrate when theyare used in surface coatings. These derivatives are furtherdistinguished by a good solubility in various surface coatings and donot have a detrimental effect on the adhesion of the surface coating tothe metallic substrate.

The invention therefore relates to a surface coating containing, ascorrosion inhibitor, at least one compound of formula I: ##STR2## inwhich R is hydrogen, halogen, C₁ -C₁₂ alkyl, C₁ -C₄ halogenoalkyl, C₁-C₁₂ alkoxy, C₁ -C₁₂ alkylthio, phenylthio, benzylthio, C₁ -C₁₂alkylsulfonyl, phenyl, --NO₂, --CN, --COOH, --COO(C₁ -C₄ alkyl), --OH,--NH₂, --NHR₈, --N(R₈)₂, --CONH₂, --CONHR₈ or --CON(R₈)₂, R₁ ishydrogen, C₁ -C₁₂ alkyl, phenyl, phenyl substituted by halogen, C₁ -C₄alkyl, C₁ -C₄ alkoxy or --NO₂, pyridyl, thienyl or furyl, R₂ ishydrogen, C₁ -C₄ alkyl or phenyl, R₃ and R₄ independently of the otherare H, C₁ -C₂₀ alkyl, C₁ -C₂₀ alkoxy, --NO₂, --CN, --COOH, --COO(C₁ -C₄alkyl), phenyl, halogen or a group of formula II: ##STR3## or R₃ and R₄together are a group --CH═CH--CH═CH--, R₅, R₆ and R₇ are hydrogen orhalogen and R₈ is C₁ -C₁₂ alkyl, C₃ -C₁₂ alkyl interrrupted by one ormore --O--, C₅ -C₈ cycloalkyl, benzyl, phenyl or phenyl substituted byhalogen, C₁ -C₄ alkyl, C₁ -C₄ alkoxy or --NO₂, or --N(R₈)₂ is apyrrolidino, piperidino or morpholino group.

In formula I, R, R₁, R₂, R₃, R₄ or R₈ as alkyl can be unbranched orbranched alkyl, for example methyl, ethyl, n-propyl, i-propyl, n-butyl,sec-butyl, i-butyl, t-butyl, n-pentyl, sec-pentyl, n-hexyl,2-ethylbutyl, n-octyl, 2-ethylhexyl, i-octyl, n-decyl, n-dodecyl,sec-dodecyl, n-tetradecyl, n-hexadecyl, n-octadecyl or eicosyl.

R₈ as alkyl interrupted by --O-- can be e.g. 2-methoxyethyl,2-ethoxyethyl, 2-butoxyethyl, 2-methoxypropyl, 3,6-dioxaheptyl,3,6-dioxaoctyl or 3,6,9-trioxadecyl.

R as halogenoalkyl can be e.g. chloromethyl, trichloromethyl,trifluoromethyl, pentafluoroethyl or nonafluorobutyl.

R₈ as cycloalkyl can be e.g. cyclopentyl, cyclohexyl or cyclooctyl. R asalkoxy can be e.g. methoxy, ethoxy, isopropoxy, butoxy, hexyloxy,octyloxy or dodecyloxy. R₃ and R₄ as alkoxy can also be e.g.tetradecyloxy, hexadecyloxy or octadecyloxy. R as alkylthio can be e.g.methylthio, ethylthio, t-butylthio, octylthio or dodecylthio.

R₁ and R₈ as phenyl substituted by halogen, C₁ -C₄ alkyl, C₁ -C₄ alkoxyor --NO₂ can be e.g. tolyl, xylyl, isopropylphenyl, t-butylphenyl,chlorophenyl, dichlorophenyl, fluorophenyl, methoxyphenyl, ethoxyphenyl,butoxyphenyl or nitrophenyl.

If R₃ and R₄ together are a group --CH═CH--CH═CH--, they form a naphthylgroup, which can be an α- or β-naphthyl group, together with the phenylgroup to which they are bonded.

Preferred corrosion inhibitors are those of formula I in which R ishydrogen, C₁ -C₄ alkyl, --CF₃, C₁ -C₄ alkoxy, halogen, --NO₂, --COOH or--COO(C₁ -C₄ alkyl), R₁ is hydrogen, C₁ -C₄ alkyl, phenyl or tolyl, R₂is hydrogen, C₁ -C₄ alkyl or phenyl, R₃ and R₄ independently of theother are H, C₁ -C₄ alkyl, C₁ -C₄ alkoxy, --NO₂, --CN, --COO(C₁ -C₄alkyl), halogen or a group of formula II, or R₃ and R₄ together are agroup --CH═CH--CH═CH--, and R₅, R₆ and R₇ are hydrogen or halogen.

Especially preferred corrosion inhibitors of formula I are those inwhich R is hydrogen, C₁ -C₄ alkyl, C₁ -C₄ alkoxy or halogen, R₁ ishydrogen, C₁ -C₄ alkyl or phenyl, R₂ is hydrogen or phenyl, R₃ and R₄independently of the other are H, C₁ -C₄ alkyl, C₁ -C₄ alkoxy, halogenor a group of formula II, or R₃ and R₄ together are a group--CH═CH--CH═CH--, and R₅, R₆ and R₇ are hydrogen or fluorine.

Preferred compounds of formula I are those in which R is hydrogen andthose in which R₁ is hydrogen or methyl and R₂ is hydrogen.

Examples of compounds of formula I are:

2-benzylthiobenzothiazole,

2-(2-chlorobenzylthio)benzothiazole,

2-(4-chlorobenzylthio)benzothiazole,

2-(2-bromobenzylthio)benzothiazole,

2-(4-bromobenzylthio)benzothiazole,

2-(2-fluorobenzylthio)benzothiazole,

2-(4-fluorobenzylthio)benzothiazole,

2-(2,4-dichlorobenzylthio)benzothiazole,

2-(2,4-dibromobenzylthio)benzothiazole,

2-(2,3,4,5,6-pentachlorobenzylthio)benzothiazole,

2-(2,3,4,5,6-pentabromobenzylthio)benzothiazole,

2-(2,3,4,5,6-pentafluorobenzylthio)benzothiazole,

2-(2-methylbenzylthio)benzothiazole,

2-(4-methylbenzylthio)benzothiazole,

2-(2-methoxybenzylthio)benzothiazole,

2-(4-methoxybenzylthio)benzothiazole,

2-(2-nitrobenzylthio)benzothiazole,

2-(4-nitrobenzylthio)benzothiazole,

2-(2-cyanobenzylthio)benzothiazole,

2-(4-cyanobenzylthio)benzothiazole,

2-(diphenylmethylthio)benzothiazole,

2-(triphenylmethylthio)benzothiazole,

2-(1-naphthylmethylthio)benzothiazole,

2-(2-naphthylmethylthio)benzothiazole,

6-chloro-2-(benzylthio)benzothiazole and

5-ethoxy-2-(benzylthio)benzothiazole.

Some of the compounds of formula I are known and some are novel. Theknown compounds of formula I are those in which R is hydrogen, chlorine,methyl, methoxy or amino, R₁ and R₂ are hydrogen, R₃ and R₄ are hydrogenor chlorine and R₅, R₆ and R₇ are hydrogen.

The invention therefore further relates to compounds of formula I:##STR4## in which R is hydrogen, halogen, C₁ -C₁₂ alkyl, C₁ -C₄halogenoalkyl, C₁ -C₁₂ alkoxy, C₁ -C₁₂ alkylthio, phenylthio,benzylthio, C₁ -C₁₂ alkylsulfonyl, phenyl, --NO₂, --CN, --COOH, --COO(C₁-C₄ alkyl), --OH, --NH₂, --NHR₈, --N(R₈)₂, --CONH₂, --CONHR₈ or--CON(R₈)₂, R₁ is hydrogen, C₁ -C₁₂ alkyl, phenyl, phenyl substituted byhalogen, C₁ -C₄ alkyl, C₁ -C₄ alkoxy or --NO₂, pyridyl, thienyl orfuryl, R₂ is hydrogen, C₁ -C₄ alkyl or phenyl, R₃ and R₄ independentlyof the other are H, C₁ -C₂₀ alkyl, C₁ -C₂₀ alkoxy, --NO₂, --CN, --COOH,--COO(C₁ -C₄ alkyl), phenyl, halogen or a group of formula II: ##STR5##or R₃ and R₄ together are a group --CH═CH--CH═CH--, R₅, R₆ and R₇ arehydrogen or halogen and R₈ is C₁ -C₁₂ alkyl, C₃ -C₁₂ alkyl interruptedby one or more --O--, C₅ -C₈ cycloalkyl, benzyl, phenyl or phenylsubstituted by halogen, C₁ -C₄ alkyl, C₁ -C₄ alkoxy or --NO₂, or--N(R₈)₂ is a pyrrolidino, piperidino or morpholino group, with theexception of the compounds of formula I in which R is hydrogen,chlorine, methyl, methoxy or amino, R₁ and R₂ are hydrogen, R₃ and R₄are hydrogen or chlorine and R₅, R₆ and R₇ are hydrogen.

Of these compounds of formula I, it is preferred to use

a) those in which R₁ is hydrogen, C₁ -C₄ alkyl or phenyl and R₂ is C₁-C₄ alkyl or phenyl,

b) those in which R is --CF₃, --NO₂, --COOH or --COO(C₁ -C₄ alkyl), and

c) those in which R₃ and R₄ independently of the other are C₁ -C₄ alkyl,C₁ -C₄ alkoxy, --NO₂, --CN, --Br, --F or a radical of formula II, or R₃and R₄ together are a radical --CH═CH--CH═CH--.

The compounds can be prepared in a manner known per se by reacting asodium mercaptide of formula III with a benzyl halide of formula IVaccording to the following equation: ##STR6## in which X is chlorine orbromine. The reaction is preferably carried out in a polar solvent, e.g.methanol, ethanol, isopropanol or dimethylformamide.

The compounds of formula I are effective corrosion inhibitors in surfacecoatings, examples of surface coatings being lacquers, paints orvarnishes. They always contain a film-forming binder in addition toother optional components.

Examples of surface coatings are those based on an alkyd, acrylic,melamine, polyurethane, epoxy or polyester resin or mixtures of suchresins. Further examples of binders are vinyl resins such as polyvinylacetate, polyvinylbutyral, polyvinyl chloride and vinyl chloridecopolymers, cellulose esters, chlorinated rubbers, phenolic resins,styrene/butadiene copolymers and drying oils. Especially preferredsurface coatings are those based on an aromatic epoxy resin.

The following are examples of surface coatings with special binders:

1. lacquers based on cold- or hot-crosslinking alkyd, acrylate,polyester, epoxy or melamine resins or mixtures of such resins, ifnecessary with the addition of an acid curing catalyst;

2. two-component polyurethane lacquers based on acrylate, polyester orpolyether resins containing hydroxyl groups and on aliphatic or aromaticpolyisocyanates;

3. one-component polyurethane lacquers based on blocked polyisocyanateswhich are unblocked during baking;

4. two-component lacquers based on (poly)ketimines and aliphatic oraromatic polyisocyanates;

5. two-component lacquers based on (poly)ketimines and an unsaturatedacrylate resin, a polyacetoacetate resin or a methylmethacrylamidoglycolate;

6. two-component lacquers based on polyacrylates containing carboxyl oramino groups and on polyepoxides;

7. two-component lacquers based on acrylate resins containing anhydridegroups and on a polyhydroxy or polyamino component;

8. two-component lacquers based on (poly)oxazolidines and acrylateresins containing anhydride groups, unsaturated acrylate resins oraliphatic or aromatic polyisocyanates;

9. two-component lacquers based on unsaturated polyacrylates andpolymalonates;

10. thermoplastic polyacrylate lacquers based on thermoplastic acrylateresins or co-reacting acrylate resins in combination with etherifiedmelamine resins; and

11. lacquer systems based on siloxane-modified acrylate resins.

The surface coatings can be pigmented or unpigmented. The pigments canbe inorganic or organic pigments or metallic pigments. Metallicpigments, e.g. aluminium pigments, are protected against corrosion bythe presence of the compounds of formula I.

The surface coatings can contain an organic solvent or they can besolventless or water-based. The surface coatings can also beradiation-curable. In this case, the binder consists of monomeric oroligomeric compounds which contain ethylenic double bonds and areconverted to a crosslinked high-molecular form on irradiation withactinic light or electron beams.

The surface coatings can contain further additives, e.g. fillers, flowcontrol agents, dispersants, thixotropic agents, adhesion promoters,antioxidants, light stabilizers or curing catalysts. They can alsocontain other known anticorrosive agents, for example anticorrosivepigments such as pigments containing phosphate or borate or metal oxidepigments, or other organic or inorganic corrosion inhibitors, e.g.nitroisophthalic acid salts, phosphoric acid esters, technical-gradeamines or substituted benzotriazoles.

It is also advantageous to add basic fillers or pigments which have asynergistic effect on corrosion inhibition in particular binder systems.Examples of such basic fillers and pigments are calcium or magnesiumcarbonate, zinc oxide, zinc carbonate, zinc phosphate, magnesium oxide,aluminium oxide, aluminium phosphate or mixtures thereof. Examples ofbasic organic pigments are those based on aminoanthraquinone.

Either the corrosion inhibitor can be added to the surface coatingduring its preparation, e.g. during dispersion of the pigment bygrinding, or the inhibitor is first dissolved in a solvent and thesolution is then stirred into the coating composition. The inhibitor isconveniently used in an amount of 0.1 to 20% by weight, preferably 0.5to 5% by weight, based on the solids content of the surface coating. Inparticular cases, it may be advantageous to add several compounds offormula I.

The surface coatings are preferably used as primers for metallicsubstrates such as iron, steel, copper, zinc or aluminium. The surfacecoatings are preferably used in aqueous systems, in particular aselectrophoretic enamels which can be deposited cathodically.

The surface coatings can be applied to the substrate by the conventionalprocesses such as spraying, dipping, painting or electrodeposition, e.g.cathodic dip-coating. Several coats are often applied. The corrosioninhibitors are added primarily to the base coat because they actprincipally at the interface between metal and surface coating. However,it is also possible to add the inhibitors to the top coat orintermediate coat as well, where they are available as a reserve.Depending on whether the binder is a physically drying resin or a heat-or radiation-curable resin, curing is carried out at room temperature orby heating (baking) or irradiation. The following Examples describe thepreparation of specific compounds of formula I and their use. Parts andpercentages are by weight.

EXAMPLE 1 2-(Benzylthio)benzothiazole

A solution of 28.1 g (0.2 mol) of benzyl chloride in 50 ml of ethanol isadded slowly, with stirring, to a solution of 33.4 g (0.2 mol) of2-mercaptobenzothiazole and 8 g (0.2 mol) of NaOH in 150 ml of ethanoland 25 ml of water. The resulting solution is refluxed for 2.5 h. Aftercooling, it is filtered and evaporated to leave 35.1 g of a yellowishsolid melting at 39°-40° C. ¹ H NMR (CDCl₃): δ4.41 (2H), δ6.98-7.8 (9H)ppm.

EXAMPLES 2-12 Preparation of analogous benzothiazoles

The following compounds are prepared analogously to Example 1:

    __________________________________________________________________________    Example                                                                            Formula                          M.p.                                                                              .sup.1 H-NMR (CDCl.sub.3)           __________________________________________________________________________          ##STR7##                        183-5°                                                                     4,40 (2H), 6,65-7,85 8 (H) ppm      3                                                                                   ##STR8##                        51-2°                                                                      4,61 (2H), 7,05-7,97 (4H) ppm       4                                                                                   ##STR9##                         49-51°                                                                    2,25 (3H), 4,50 (2H), 7,0-7,98                                                (8H) ppm                            5                                                                                   ##STR10##                       67-8°                                                                      3,75 (3H), 4,50 (2H), 6,77-7,92                                               (8H) ppm                            6                                                                                   ##STR11##                       97-8°                                                                      6,30 (1H), 7,10-7,90 (14H) ppm      7                                                                                   ##STR12##                       101-2°                                                                     7,14-7,55 (19H) ppm                 8                                                                                   ##STR13##                       75-6°                                                                      4,92 (2H), 6,99-7,99 (11H) ppm      9                                                                                   ##STR14##                       72-3°                                                                      4,35 (2H), 6,82-7,63 (8H) ppm       10                                                                                  ##STR15##                       76-8°                                                                      1,15 (3H), 3,80 (2H) 4,36 (2H),                                               6,60-7,59 (8H)                      11                                                                                  ##STR16##                       104-6°                                                                     4,59 (4H), 7,21-8,0 (12H) ppm       12                                                                                  ##STR17##                       Oil 1,55 (3H), 4,76 (1H), 6,68-7,65                                               (9H) ppm                            __________________________________________________________________________

EXAMPLE 13 Anticorrosive alkyd resin lacquer

The lacquer is prepared by mixing the following components:

40 parts of Alphthalat® AM 380 alkyd resin (60% xylene solution) fromReichhold Albert Chemie AG

10 parts of iron oxide red 225 from Bayer AG

13.6 parts of talc (micronized)

13 parts of micronized calcium carbonate (Millicarb®, Pluss-Staufer AG)

0.3 part of Luaktin® antiskinning agent (BASF)

0.6 part of 8% cobalt naphthenate solution

24.7 parts of 6:40 xylene/propylene glycol monomethyl ether mixture.

The corrosion inhibitors shown in the following Table are firstdissolved in part of the solvent and then added to the lacquer. Thelacquer is ground with glass beads for 7 days until the particle size ofthe pigment and fillers is less than 15 μm.

The lacquer is sprayed on to 7×13 cm sandblasted steel sheets as a coatwhich is ca. 50 μm thick after drying. After drying for 7 days at roomtemperature, the test pieces are postcured for 60 minutes at 60° C.

Two 4 cm long cruciform cuts are made in the cured surface of thelacquer, down to the metal, using a Bonder cross-cutter. The edges areprotected by the application of an edge protection agent (Icosit® 255).

The test pieces are then subjected to a salt spray test according toASTM B 117 for a period of 600 hours. After every 200 hours ofweathering, the condition of the surface coating is assessed in respectof the degree of blistering (according to DIN 53 209) at the cross-cutand over the lacquered area and the degree of rusting (according to DIN53 210) over the whole area.

When the test is complete, the surface coating is removed by treatmentwith concentrated sodium hydroxide solution and the corrosion of themetal is assessed at the cross-cut (according to DIN 53 167) and overthe remaining area. Assessment is made in each case on a 6-point scale.The sum of the evaluation of the surface coating and the evaluation ofthe metal surface gives the anticorrosion value AC: the higher thisvalue, the more effective the tested inhibitor.

    ______________________________________                                        Corrosion inhibitor                                                                       Amount   Evaluation                                                                              Evlauation                                                                            AC                                     ______________________________________                                        none        --       1,8       0,6     2,4                                    Example 1   2%       4,9       3,2     8,1                                    Example 2   2%       3,7       4,3     8,0                                    Example 3   2%       5,3       4,6     9,9                                    Example 4   2%       2,6       2,6     5,2                                    Example 5   2%       3,3       2,5     5,8                                    Example 6   2%       3,8       3,5     7,3                                    Example 7   2%       3,4       3,7     7,1                                    Example 8   2%       3,2       3,0     6,2                                    Example 9   2%       4,5       4,1     8,6                                     Example 10 2%       3,9       3,3     7,2                                     Example 11 2%       3,2       3,0     6,2                                     Example 12 2%       5,1       4,0     9,1                                    ______________________________________                                    

EXAMPLE 14 Anticorrosive electrophoretic enamel

217.8 g of an electrophoretic enamel are mixed with 1.5 g of propyleneglycol monophenyl ether, 7.5 g of lactic acid (88%) and 1.5 g of anon-ionic wetting agent (X-Blend, Du Pont), with stirring. Theelectrophoretic enamel is a solution of an aromatic epoxy resincontaining amino groups and hydroxyl groups and contains a cappeddiisocyanate as crosslinking agent. The enamel is a product from Du Pontde Nemour and has a solids content of ca. 36%.

10 g of dibutyltin dilaurate, as crosslinking catalyst, and 4 g of thecorrosion inhibitor of Example 12 (2-(1-phenylethylthio)benzothiazole)are added to the above mixture and the resulting mixture is stirred togive a homogeneous solution. This corresponds to a content of 4.2% ofcorrosion inhibitor, based on solids.

76 g of water are initially added, with stirring. After 20 minutes, afurther 76 g of water are added. After stirring for 30 minutes, afurther 48 g of water are added. The emulsion formed is stirred for 48hours and then made up with water to a volume of 1000 ml. The resultingbath has a solids content of ca. 10%. It has a pH of 4.9 and theconductivity is 2000 μS.

15×7.5 cm steel sheets are dipped into this bath as the cathode. At abath temperature of 29° C., a voltage of 220 volts is applied for 120seconds to deposit the enamel. The sheets are then rinsed with water,dried briefly and baked for 30 minutes at 176° C. The resulting curedenamel film has a thickness of ca. 24 μm.

For corrosion testing, a 70×0.5 mm cut is made in the surface of thesheets, which is subjected to a GM Scab Test (TM 54-26).

A test cycle consists of immersion of the test pieces for 15 minutes ina 5% aqueous NaCl solution, followed by storage for 75 minutes at roomtemperature and 22.5 hours in a humidity cabinet (Hot Pack Model 417522)at 60° C. and 85% relative humidity. After 5 such daily cycles, the testpieces are stored for a further 2 days in the humidity cabinet.

The sheets are then rinsed with water. The enamel which is no longeradhering firmly, due to corrosion, is mechanically removed and theaverage width of the corrosion zone at the cut is measured.

The average width of the corrosion zone is 7 mm for the test piecescontaining the corrosion inhibitor. The average width of the corrosionzone for control test pieces without corrosion inhibitor is 31 mm.

We claim:
 1. A surface coating based on an alkyd, acrylic, melamine,polyurethane, epoxy or polyester resin, mixtures of such resins, vinylresins, cellulose esters, chlorinated rubbers, phenolic resins,styrene/butadiene copolymers or drying oils, containing, as corrosioninhibitor, at least one compound of the formula I: ##STR18## in which Ris hydrogen, halogen, C₁ -C₁₂ alkyl, C₁ -C₄ halogenoalkyl, C₁ -C₁₂alkoxy, C₁ -C₁₂ alkylthio, phenylthio, benzylthio, C₁ -C₁₂alkylsulfonyl, phenyl, --NO₂, --CN, --COOH, --COO(C₁ -C₄ alkyl), --OH,--NH₂, --NHR₈, --N(R₈)₂, --CONH₂, --CONHR₈ or --CON(R₈)₂, R₁ ishydrogen, C₁ -C₁₂ alkyl, phenyl, phenyl substituted by halogen, C₁ -C₄alkyl, C₁ -C₄ alkoxy or --NO₂, pyridyl, thienyl or furyl, R₂ ishydrogen, C₁ -C₄ alkyl or phenyl, R₃ and R₄ independently of the otherare H, C₁ -C₂₀ alkyl, C₁ -C₂₀ alkoxy, --NO₂, --CN, --COOH, --COO(C₁ -C₄alkyl), phenyl, halogen or a group of formula II: ##STR19## or R₃ and R₄together are a group --CH═CH--CH═CH--, R₅, R₆ and R₇ are hydrogen orhalogen and R₈ is C₁ -C₁₂ alkyl, C₃ -C₁₂ alkyl interrupted by one ormore --O--, C₅ -C₈ cycloalkyl, benzyl, phenyl or phenyl substituted byhalogen, C₁ -C₄ alkyl, C₁ -C₄ alkoxy or --NO₂, or --N(R₈)₂ is apyrrolidino, piperidino or morpholino group.
 2. A surface coatingaccording to claim 1 containing at least one compound of formula I inwhich R is hydrogen, C₁ -C₄ alkyl, --CF₃, C₁ -C₄ alkoxy, halogen, --NO₂,--COOH or --COO(C₁ -C₄ alkyl), R₁ is hydrogen, C₁ -C₄ alkyl, phenyl ortolyl, R₂ is hydrogen, C₁ -C₄ alkyl or phenyl, R₃ and R₄ independentlyof the other are H, C₁ -C₄ alkyl, C₁ -C₄ alkoxy, --NO₂, --CN, --COO(C₁-C₄ alkyl), halogen or a group of formula II, or R₃ and R₄ together area group --CH═CH--CH═CH--, and R₅, R₆ and R₇ are hydrogen or halogen. 3.A surface coating according to claim 1 containing at least one compoundof formula I in which R is hydrogen, C₁ -C₄ alkyl, C₁ -C₄ alkoxy orhalogen, R₁ is hydrogen, C₁ -C₄ alkyl or phenyl, R₂ is hydrogen orphenyl, R₃ and R₄ independently of the other are H, C₁ -C₄ alkyl, C₁ -C₄alkoxy, halogen or a group of formula II, or R₃ and R₄ together are agroup --CH═CH--CH═CH--, and R₅, R₆ and R₇ are hydrogen or fluorine.
 4. Asurface coating according to claim 1 containing at least one compound offormula I in which R is hydrogen.
 5. A surface coating according toclaim 1 containing at least one compound of formula I in which R₁ ishydrogen or methyl and R₂ is hydrogen.
 6. A surface coating according toclaim 1 containing 0.5 to 5% by weight, based on the solids content ofthe surface coating, of at least one compound of formula I.
 7. A surfacecoating according to claim 1 which is a primer for metallic substrates.8. A surface coating according to claim 7 which is a primer for iron,steel, copper, zinc or aluminium.
 9. A surface coating according toclaim 1 which is water-based.
 10. A surface coating according to claim 9which can be deposited cathodically.
 11. A surface coating according toclaim 1 based on an alkyd, acrylic, melamine, polyurethane, epoxy orpolyester resin or a mixture of such resins.